In typical land line systems, remote stations and control centers are connected by copper or fiber optic circuits which have a data throughput capacity and performance integrity that is generally significantly better than the data throughput capacity and performance integrity provided by the air interface in a cellular telephone system. As a result, the conciseness of overhead required to manage any selected communication link protocol for land line systems is of secondary importance.
In cellular telephone systems, an air interface communications link protocol is required in order to allow a mobile station to communicate with a cellular switching system (CSS). The communications link protocol is used to initiate and to receive cellular telephone calls. The electromagnetic spectrum available for use by cellular telephone systems is limited and is partitioned into units called channels. Individual channels are used as communication links on either a shared basis or on a dedicated basis. When individual channels are used as communication links on a shared basis, multiple mobile stations may either listen to or contend for the same channels. For the contending situation, each shared channel can be used by a plurality of mobile stations which compete to obtain exclusive use of the channel for a limited period of time. On the other hand, when individual channels are used as communication links on a dedicated basis, a single mobile station is assigned the exclusive use of the channel for as long as it needs it.
In light of the generally reduced data throughput capacity and performance integrity afforded by an individual channel in a channel sharing situation in a cellular telephone environment, the selection of an efficient air interface protocol to serve as the basis of the communication link becomes paramount.
The communication link protocol is commonly referred to as a layer 2 protocol within the communications industry and its functionality includes the delimiting or framing of higher layer messages. Traditional layer 2 protocol framing mechanisms of bit stuffing and flag characters are commonly used in land line networks today to frame higher layer messages, which are referred to as layer 3 messages. These layer 3 messages may be sent between communicating layer 3 peer entities residing within mobile stations and cellular switching systems.
Communications between mobile stations and the cellular switching system will in general be described with reference to FIGS. 1 and 2. FIG. 1 illustrates a plurality of layer 3 messages 11, layer 2 frames 13, and channel bursts 15. A channel burst 15 is the shortest or smallest transmission event that occurs between the mobile stations and the CSS. In FIG. 1, the channel bursts are separated in time but they do not need to be separated. In other words, as soon as one channel burst ends the next channel burst could begin. A channel burst 15 contains a complete layer 2 frame as well as other information such as, for example, error correction information. Each layer 2 frame is divided into a plurality of different fields. One of the fields, the data field which has a limited length, contains at least a portion of a layer 3 message 11. Since layer 3 messages have variable lengths depending upon the amount of information contained in the layer 3 message, a plurality of layer 2 frames may be needed for transmission of a single layer 3 message. As a result, a plurality of channel bursts may also be needed to transmit the entire layer 3 message as there is a one to one correspondence between channel bursts and layer 2 frames. When multiple channel bursts are required to send a layer 3 message, the multiple bursts are not usually successive bursts. Since time is required to receive, process, and react to a received burst, the bursts required for transmission of a layer 3 message are sent in a staggered format as is illustrated in FIG. 2. In FIG. 2, a path A illustrates the communications between a mobile station A and the CSS. In this example, the mobile station A uses every sixth uplink (the mobile station to CSS direction) channel burst to transmit a layer 3 message to the CSS. As a result, other mobile stations, for example, mobile station B, can also transmit a layer 3 message to the CSS while the mobile station A is in the process of transmitting a layer 3 message by using a different set of associated uplink bursts.
In Time Division Multiple Access (TDMA) based cellular telephone systems, the inherent channel coding format has been proposed as a means to accomplish the basic layer 2 frame delimiting function and is an effective way of eliminating the previously referenced traditional layer 2 framing mechanisms of bit stuffing and flag characters. The TDMA channels can be used on either a dedicated or a shared basis. These TDMA channels are essentially a sequence of transmission units or bursts wherein each burst has a certain fixed information carrying capacity. As a result, each burst typically carries only a portion of a layer 3 message. When a TDMA channel is used on a shared basis, it can be referred to as a digital control channel in which multiple mobile stations either contend for its use when sending layer 3 messages to the cellular switching system (on the uplink) or collectively listen to the CSS for potential layer 3 messages intended for their reception (on the downlink). In the uplink direction, multiple mobile stations attempt to communicate with the cellular switching system on a contention basis, while multiple mobile stations listen for layer 3 messages sent from the cellular switching system in the downlink direction. However, additional layer 2 throughput performance enhancements are still desirable as both the uplink or random access channel (RACH) and the downlink or access response channel (ARCH) may have multiple distinct layer 3 messages pending at any given point of time.
In known systems, any given layer 3 message must be carried using as many TDMA channel bursts as required to send the entire layer 3 message. The last TDMA channel burst used in sending a distinct layer 3 message may not be used to its fullest capacity since the last portion of the layer 3 message may not occupy the entire data field. In these systems, no provisions have been made for allowing any pending layer 3 message to be started within the remaining capacity of the data field. As a result, the present systems effectively waste available TDMA channel burst capacity whenever an additional separate and distinct layer 3 message is available and ready for transmission at the same time the completion of a previously initiated layer 3 message transmission can be accomplished within any given TDMA channel burst.